1. Field of the Invention
The present invention generally relates to a stepping motor driving apparatus for driving and controlling a stepping motor for indexing or positioning a member under control to a target or desired position. Further, the invention is concerned with an exhaust gas recirculation control system in which a stepping-motor-driven type exhaust gas recirculation valve is employed for recirculating an exhaust gas of an internal combustion engine. More particularly, the invention is concerned with an exhaust gas recirculation (EGR) control system which can ensure improved response performance as well as enhanced control accuracy in positioning an exhaust gas recirculation control valve.
2. Description of the Related Art
Heretofore, in the field of engine control systems for automobiles or motor vehicles, the exhaust gas recirculation control technique for feeding back or recirculating a part of exhaust gas to the engine for the purpose of suppressing NO.sub.x -components contained in the engine exhaust gas by lowering the combustion temperature is well known. For better understanding of the invention, description will first be made of a conventional exhaust gas recirculation control system known heretofore by reference to FIG. 5.
Referring to FIG. 5, an internal combustion engine system is comprised of an engine proper 1 (hereinafter referred to as an engine) having a plurality of cylinders, an air cleaner 2 for purifying intake air to be introduced into the engine 1, an intake air pipe 3 for feeding the air introduced through the air cleaner 2 to the engine 1, an intake manifold 4 for connecting the intake pipe 3 to the plurality of cylinders of the engine 1, a fuel injector 5 for injecting fuel into the intake pipe 3, a pressure sensor 6 for detecting a pressure P within the intake pipe 3, a throttle valve 7 disposed within the intake pipe 3 for controlling an intake air flow, a throttle position sensor 8 for detecting an opening degree .theta. of the throttle valve 7, and a bypass air flow rate control means 9 for controlling an air flow rate which bypasses the throttle valve 7 via a pipe connected in parallel to the throttle valve 7.
An exhaust gas recirculation pipe 10 is provided for the purpose of feeding back or recirculating a part of the exhaust gas discharged from the engine 1 to the intake side thereof. An exhaust gas recirculation valve 11 of a stepping-motor-driven type is installed in the pipe 10 for controlling the flow rate of the exhaust gas recirculated therethrough. The exhaust gas recirculation control valve 11 constitutes an exhaust gas recirculation flow control means for controlling the exhaust gas recirculation flow rate in dependence on the operation states of the engine 1.
An ignition coil 13 serves for generating a high voltage required for combustion of air/fuel mixture gas within the individual cylinders of the engine 1. Provided in association with the ignition coil 13 is a firing or ignitor circuit 14 for interrupting a primary current of the ignition coil 13, to thereby generate a spark for triggering combustion of the air/fuel mixture. The exhaust gas resulting from the combustion is discharged through an exhaust pipe 15. A catalytic converter 16 is installed in the exhaust pipe 15 in a downstream region thereof.
An ignition signal Q generated by the ignition coil 13, as driven by the ignitor 14, has a frequency corresponding to the rotation speed (rpm) of the engine 1 and thus can be utilized as a sensor signal indicative of the rotation number (rpm) of the engine 1.
A water temperature sensor 17 for detecting temperature T of cooling water of the engine 1 cooperates with the throttle position sensor 8, the ignition coil 13 and others to constitute a sensor means which provides information concerning the operation states of the engine 1. An ignition key switch 21 is closed upon starting of the engine operation for supplying an electric power to various electric units of the motor vehicle from an onboard battery 20.
An electronic control unit (ECU) 22 which is put into operation upon reception of an electric power from the battery 20 includes a fuel injection amount arithmetic means and an exhaust gas recirculation flow arithmetic means which fetch from a variety of sensor means the engine operation state information such as the throttle opening degree .theta., the intake pipe pressure P, the ignition signal Q and others to thereby determine arithmetically or calculate the fuel injection amount and the exhaust gas recirculation flow rate, respectively, whereby a fuel injection control signal J for the fuel injector 5 and an exhaust gas recirculation control signal C for the exhaust gas recirculation control valve 11 are outputted from the electronic control unit 22.
FIG. 6 is a block diagram showing in detail a configuration of the electronic control unit 22 of FIG. 5. Referring to FIG. 6, a microcomputer 100 includes a CPU (Central Processing Unit) 200 for generating the fuel injection control signal J for the fuel injector 5 and the exhaust gas recirculation control signal C for the exhaust gas recirculation control valve 11 as determined on the basis of the aforementioned engine operation state information Q, P, .theta. and T in accordance with a predetermined program, a free-running counter 201 for measuring a rotation period of the engine 1, a timer 202 for measuring time spans for various controls, an analogue-to-digital converter (hereinafter referred to as the A/D converter) 203 for converting analogue input signals into digital signals, a RAM (Random Access Memory) 205 used as a work memory, a ROM (Read-Only Memory) 206 for storing various operation programs, an output port 207 for outputting the fuel injection control signal J and the exhaust gas recirculation control signal C and a common bus 208 for interconnection between the CPU 200 and the various elements 201 to 207 mentioned above.
The electronic control unit 22 further includes a first input interface circuit 101 for shaping the ignition signal Q from the ignition coil 13 to thereby generate an interrupt signal to be inputted to the microcomputer 100. Upon generation of this interrupt signal, the CPU 200 incorporated in the microcomputer 100 reads the count value from the counter 201 to thereby calculate the rotation period of the engine 1 on the basis of a difference between the count values read out at the instant time point and at a preceding time point, respectively. The engine rotation period thus determined is then stored in the RAM 205.
The electronic control unit 22 includes a second input interface circuit 102 which serves for fetching the intake pipe pressure P, the throttle opening degree .theta. and the cooling water temperature T from the pressure sensor 6, the throttle position sensor 8 and the water temperature sensor 17, respectively. The sensor signals mentioned above are inputted to the A/D converter 203. On the other hand, an output interface circuit 104 serves to supply the fuel injection control signal J and the exhaust gas recirculation control signal C supplied through the output port 207 to the fuel injector 5 and the exhaust gas recirculation control valve (hereinafter also referred to as the EGR control valve) 11, respectively, after amplification.
FIG. 7 is an elevational view showing a structure of the EGR control valve unit 11. Referring to the figure, a unipolar type stepping motor 30 is mounted in a valve casing of the EGR control valve 11 at a top end thereof for controlling or indexing the EGR control valve 11 between the fully closed position and the fully opened position stepwise over forty-eight steps (increments). The stepping motor 30 has an output shaft 31 with which a feed screw 32 is interlocked so as to be displaced upwardly or downwardly in correspondence to rotation of the motor shaft 31. A valve stem 33 is driven upwardly or downwardly, as viewed in the figure, by means of the feed screw 32 for regulating the flow cross-section area of the EGR control valve 11. A compression or coil-spring 34 urges constantly the valve stem 33 in the upward or valve opening direction. Disposed between the motor shaft 31 of the stepping motor 30 and the feed screw 32 is a translation mechanism 35 which serves to translate the rotation of the motor shaft 31 into the vertical (up-and-down) displacement of the motor shaft 31.
FIG. 8 is a characteristic diagram for illustrating a relation between the flow rate (liter per minute) of the EGR control valve 11 and the number of steps over which the stepping motor 30 is operated, wherein the step number of the stepping motor 30 is taken along the abscissa with the exhaust gas recirculation flow rate being taken along the ordinate. As can be seen in FIG. 8, the EGR control valve 11 assumes the fully closed state, when the step number of the stepping motor 30 is zero while assuming the fully opened state when the step number of the stepping motor 30 is "48".
It can further be seen from FIG. 8 that in the fully opened state of the EGR control valve 11 (e.g. when the exhaust gas recirculation flow rate is 500 liters per minute), a pressure difference .DELTA.P of 200 mmHg prevails between the entrance and the exit of the EGR control valve 11. It goes without saying that the pressure difference .DELTA.P across the EGR control valve 11 becomes higher than 200 mmHg in the fully closed state.
FIGS. 9 and 10 are flow charts for illustrating operations of the CPU 200 of the conventional exhaust gas recirculation control system, wherein FIG. 9 shows a main routine while FIG. 10 shows an exhaust gas recirculation control routine. In the following, operations of the conventional exhaust gas recirculation control system shown in FIGS. 5 to 7 will be described by reference to FIGS. 8, 9 and 10.
In a control processing step S1 of the main routine, calculation of the engine rotation number Ne (rpm) on the basis of the ignition signal Q, fetching of the aforementioned various sensor signals via the A/D converter 203, the fuel control and others processings are performed. Upon completion of the control processing step S1, an exhaust gas recirculation control processing step S2 is executed, Upon completion of execution of the step S2, the step main routine S1 is resumed. Thus, through the processing steps S1 and S2, operation of the engine 1 can be controlled.
In more detail, the exhaust gas recirculation (EGR) control processing step S2 is executed in such a manner as illustrated in FIG. 10.
First, in a step S601, the engine rotation number Ne (rpm) and the intake pipe pressure P already determined in the step S1 are fetched. Subsequently, in a step S602, a desired opening degree of the EGR control valve 11 (i.e., a desired step number of the stepping motor 30) is calculated on the basis of the engine rotation number Ne and the intake pipe pressure P as fetched.
Next, correction is made on the desired or target opening degree as determined in order to take into consideration the warmed-up state of the engine 1. To this end, the cooling water temperature T already determined in the step S1 (FIG. 9) is fetched in a step S603. In succession, in a step S604, the desired or target opening degree of the EGR control valve 11 determined in the step S602 is so corrected that it is set at a smaller value when the cooling water temperature T is low.
Finally, in a step S605, the stepping motor 30 is driven so that the desired EGR valve opening degree as determined in the step S602 and corrected in the step S604 is set.
Through the processing steps S601 to S605, the exhaust gas recirculation flow rate can be controlled to be optimal.
Next, operation of the stepping motor 30 will be described in detail by reference to FIGS. 11A and 11B together with FIGS. 12A and 12B, in which FIG. 11A is a diagram for illustrating inter-phase relation of the stepping motor 30 for driving the EGR control valve 11, FIG. 11B is a view showing interconnection of the winding coils of the stepping motor, FIGS. 12A is a view showing a driving pattern of the stepping motor 30 according to a two-phase excitation driving scheme, and FIG. 12B is a view for illustrating rotating direction of the stepping motor 30.
As is shown in FIG. 11A and 11B, windings A and B are each implemented as a bifilar winding and connected in common to a positive pole of the battery 20, wherein the coils constituting each of the windings A and B are interconnected such that upon excitation of the winding, the direction of magnetic flux in one coil is opposite to that in the other coil.
When the stepping motor 30 implemented in the structure described above is driven in accordance with a two-phase excitation driving scheme illustrated in FIGS. 12A and 12B, first and second phase stator coils (indicated by 1 and 2 and hatched areas in FIG. 11A) are first excited at a step position "0". Accordingly, a magnetic pole distribution shown in FIG. 11A prevails in the stator assembly. As a consequence, the S-pole of the rotor is indexed to the step position "0" which corresponds to the center of N-pole of the stator resulting from synthesization of magnetization of the first and second phase-coils 1 and 2.
At a step position "1", magnetization of the winding A is changed over so that the first phase-coil 1 is deenergized or deexcited while a third phase-coil 3 is excited. As a consequence, the S-pole of the rotor is angularly displaced to the step position "1" which corresponds to a center of the N-pole resulting from synthesization of the magnetizations of the second and third phase-coil 2 and 3, as indicated by an arrow in FIG. 11A.
In a step position "2", the magnetization of the winding B is changed over so that the second phase-coil 2 is deexcited with the fourth phase-coil 4 being excited. Thus, the S-pole of the rotor is further displaced to the step position "2". By repeating the excitation of the coils two by two with the phase shifting in this manner, a rotating magnetic field is generated, whereby the stepwise rotation of the rotor of the stepping motor is realized. Thus, the motor shaft 31 is rotated stepwise in the counterclockwise direction, as illustrated in FIG. 12B.
Of course, by changing the excitation in the pattern opposite to that mentioned above, rotation of the motor shaft 31 can be reversed (i.e., in the clockwise direction).
As is apparent from the above, by exciting the individual coils of the stepping motor 30 at a predetermined time interval T (e.g. at 100 milliseconds equivalent to 100 pulses per second or PPS, see FIG. 12A) in the manner described above, the opening control (flow rate control) of the EGR control valve 11 can be carried out.
At this juncture, it should be mentioned that as the method of driving the stepping motor 30, there is known in addition to the aforementioned two-phase excitation driving scheme a one/two-phase excitation driving scheme according to which the one-phase excitation and the two-phase excitation are alternated.
Next, referring to FIG. 13, a driving pattern according to the one/two-phase excitation driving scheme will be elucidated.
According to the one/two-phase excitation type driving scheme, the stepping motor 30 is driven by repeating the excitation of the coils on a one-by-one basis (e.g. at A in FIG. 13) and the two-phase excitation (e.g. at B in FIG. 13) mentioned above. The one/two-phase excitation driving scheme is advantageous in that the angular distance for one step can be decreased by a half with the driving period T' therefor being 75 milliseconds, which is shorter about a half when compared with the aforementioned two-phase excitation driving scheme.
FIG. 14 is a view showing comparison between the two-phase excitation driving scheme and the one/two-phase excitation driving scheme. At first, comparison will be made as to the resolution. According to the two-phase excitation driving scheme, the rotor and hence the motor shaft is caused to shift by one full step. On the other hand, according to the one/two-phase excitation driving scheme, the rotor rotates stepwise by a distance corresponding to a half of one step. Thus, the one/two-phase excitation driving scheme can ensure a higher resolution for the positioning or indexing of the EGR control valve 11.
On the other hand, with regards to the driving period which is an index indicating response speed of the stepping motor, the two-phase excitation driving scheme can ensure 100 milliseconds at highest, while in the case of the one/two-phase excitation driving scheme, it is 75 milliseconds. To say in another way, the time required for the rotor of the stepping motor to advance for one step is 100 milliseconds in the case of the two-phase excitation while it is 150 milliseconds in the case of the one/two-phase excitation driving scheme. Thus, the former is excellent in respect to the response speed. For this reason, the two-phase excitation driving scheme has heretofore been adopted for driving the stepping motor 30 for controlling the exhaust gas recirculation control valve 11 by putting importance on the response speed.
The conventional exhaust gas recirculation control system of the structure described above thus suffers from a problem that because the exhaust gas recirculation control valve 11 of a relatively large flow capacity is driven in accordance with the two-phase excitation driving scheme with as relatively low resolution as forty-eight steps or increments from the fully closed state to the fully opened state, magnitude of change in the exhaust gas recirculation flow rate as brought about by the change of the rotor position for one step is relatively large, which means that it is difficult or practically impossible to control the exhaust gas recirculation flow rate with higher accuracy or fineness.
An approach for coping with the problem mentioned above by increasing the number of steps of the EGR control valve driving motor which is driven in accordance with the two-phase excitation driving scheme mentioned above will encounter another problem that high cost is involved because of necessity for use of the stepping motor having high resolution (i.e., a greater number of steps). On the other hand, when the EGR control valve 11 is driven in accordance with the one/two-phase excitation driving scheme with a view to increasing the number of steps intervening between the fully closed state and the fully opened state, the resolution of control can certainly be enhanced. However, there arises a problem that the response performance is degraded because of increase in the driving frequency and corresponding delay in indexinger shifting the EGR control valve 11 from one to another position.